Conventional laser receivers are particularly small hand-held devices, which include a display and a laser light photo sensor. In spite of being hand-held laser receivers, those laser receivers are often provided with an adapter in order to be mounted on a tripod or other kind of stand. The photo sensor of the laser receiver typically comprises a zero position. For considering a detected laser beam as to be on-grade, the light of said laser beam has to incident the zero position of the photo sensor. Typically, the photo sensor comprises a plurality of photo sensitive elements providing an electrical output signal when illuminated by the laser beam. Particularly, the photosensitive elements are arranged in a linear array having the zero position usually in the centre of the array. A circuitry included in the laser receiver is connected to the photo sensor and is designed to compute the electronic output signal of the photo sensor. The electronic output signal of the photo sensor indicates the incidence of the detected laser beam relative to the zero position. The circuitry is designed to derive from the electronic output signal of the photo sensor the relative position between the detected laser beam and the laser receiver. In order to find a laser beam by using a laser receiver the laser receiver is moved by the operator slowly through the space the searched laser beam is assumed to be. In case the laser beam hits the photos sensor of the laser receiver, this hit and the position of the hit with respect to the zero position is indicated on the display of the laser receiver, which is connected to the circuitry. The movements of the laser receiver are than concentrated by the operator to the part of space, where the incidence had been indicated, until the laser beam and the laser receiver are indicated on the display or by an acoustic signal as being on-grade.
Most frequently laser receivers of the above described form are used on construction sides collaborating with the rotating laser beam of a rotating construction laser. A laser system comprising such a laser receiver in collaboration with a rotating construction laser is disclosed in U.S. Pat. No. 6,314,650 B1. The laser beam or the laser plane generated by the laser transmitter can be adjusted in response to one or more inputs by a user of the system. The system is operable in different modes, such that communication from the laser receiver to the laser transmitter results in various adjustments to the laser beam or tilt or slope of the plane generated by the rotating laser beam. The system is preferably operable to automatically account for any drift of the beam or laser plane. Further the principal steps of the different modes like plane lock/grade lock (zero-drift or plane adjustment), line lock (target detection mode), laser catching and laser tracking (slope matching: the beam is adjusted to follow the movement of the laser receiver) etcetera are explained in U.S. Pat. No. 6,314,650 B1. However, any mistakes in detecting the elevation of or in adjusting the laser beam or laser plane, respectively, caused by a movement of the laser receiver are not subject matter of U.S. Pat. No. 6,314,650 B1 and are not even mentioned as being a possible problem.
As it can exemplarily be seen by means of U.S. Pat. No. 6,314,650 B1, it is usually assumed that during the collaboration of laser receiver and laser beam or laser plane, especially during grade lock/plane lock modes, the position of the laser receiver remains stable, whereas the position and direction of the emitted laser beam may drift over time, particularly due to temperature changes. The emitted laser beam is then re-adjusted for compensation of such drift, particularly for meeting the zero position of the laser receiver with the generated laser beam or laser plane, respectively. However, the correction is false when the laser receiver tilts or moves.
U.S. Pat. No. 6,292,258 B1 recognized at least that a tilt of the laser receiver can influence the precision of a detected elevation of a rotating reference laser beam. In order to solve the recognized problem U.S. Pat. No. 6,292,258 B1 provided a laser receiver equipped with a laser beam detection unit provided with a laser light photo sensor with a plurality of photosensitive elements arranged in a (linear) array, capable of delivering an elevation signal indicative of an elevation of a laser beam relative to the laser receiver, and provided with an inclination sensor providing an electronic signal in case the laser receiver tilts. Thus, the disclosed laser receiver is capable of detecting the presence and elevation of a rotating reference laser beam and of providing an indication of a laser receiver tilt. An integrated circuitry is designed for correcting a generated elevation signal from the photo sensor as a function of the indicated laser receiver tilt. Thus the laser receiver is capable to indicate the user a detected elevation of the laser beam faultless with respect to tilt-faults caused by a tilt of the laser receiver.
However, in cases, where the laser receiver shows other movements than a tilt, these other movements, i.e. sideward or upward or downward movements (sinking in mud) or a combination of these movements with a tilt can not be considered by laser receivers as disclosed in the documents cited above. A correct adjustment or re-adjustment of the laser beam or a correction of the position of the laser receiver in response to a movement other than a tilt of the laser receiver is not possible.